1. Field of the Invention
This invention relates to apparatus for reducing minimum exposing time of an image-acquiring device of an image processing system, and particularly relates to apparatus for reducing minimum scanning time necessary for a high-resolution-image-acquiring device to scan an object in a low resolution.
2. Description of the Prior Art
An image processing system makes use of focusing a reflected light beam from an object through a photodetector to generate an electrical signal representing the image of the object for further processing, storing and displaying. Among various applications such as image scanners, camera recorders or facsimile machines everywhere in the modern world, in spite of somewhat differences between these machines, there is one necessary primary step. In other words, it is necessary for an image system to acquire an image signal by converting an image of the object to an electrical signal.
Taking an image scanner as example, the block diagram in the prior art is shown in FIG. 1. It is composed of an exposing timing signal source 8, a shift control signal source 9, a light source 10, a glass surface 11, a mirror 12, a lens 13, a photo-sensing device 14, a charge coupled device (CCD) shift register 15, a pre-processing device 16 and a post-processing device 17. The pre-processing device 16 is implemented by electrically coupling a dc-gain voltage amplifier 16a, an analogue-to-digital converter (ADC) 16b. The waveform of the output signal 20, shown in FIG. 2, of an exposing timing signal source 8 is fed to photo-sensing device 14.
This system mentioned above operates in the way that photo-sensing device 14 converts the light emitted by light source 10, a text or a picture firstly reflected by the glass surface 11 and secondly reflected by the mirror 12 to an image signal.
Note that when the front edge of pulse 21 (FIG. 2) is fed to the photo-sensing device 14, the photo-sensing device 14 pour out all the charges to the CCD shift register 15. After the photo-sensing device 14 has poured out all the stored charges, it cumulate the charges produced in the time interval between the back edge of the pulse 21 and front edge of the pulse 23. Subsequently, the photo-sensing device begins to produce and accumulate charge until next front edge arrives. Thus an optical image is transformed into an electrical signal. The electrical signal parallel output to the CCD shift register 15, and is serially fed to the pre-processing device 16.
To precisely describe the operation of photo-sensing device 14 responding to the exposing timing signal source 8, and that of the CCD shift register 15 responding to the shift control signal source 9. The operation of the system is described below. A line of scanned object is exposed to the light source 10, and the photo-sensing device 14 transfers the light from the line on the scanned object into a plurality of groups of charges responding to the pulse 19 and pulse 21 of the output signal 20 of the exposing timing signal source 8. Each cell of the photo-sensing device 14 is exposed to the light from the lens 13 during the exposing time interval between pulse 21 and 23 of the output signal 20 of the exposing timing signal source 8. After the pulse 21 has arrived at the photo-sensing device 14, the plurality groups of charges is fed to the CCD shift register 15 at the same time. In addition, each of the plurality groups of charges generated by each cell of the photo-sensing device 14 is fed to the corresponding potential-energy wells of the CCD shift register 15.
Subsequently, each of the plurality groups of charges stored in each potential-energy wells in the CCD shift register 15 is transmitted to the pre-processing device 16 one after another responding to the output clock pulse 30 of the shift control signal source 9. The plurality groups of charge is stored in each potential-energy well of the CCD shift register 15 before the pulse 23 next to the pulse 21 arrive at the photo-sensing device 14. In addition, each group of charge stored in each potential-energy well of the CCD shift register 15 is subsequently transmitted to the pre-processing device 16. In other words, the group of charge stored in the first potential-energy well a1 of the CCD shift register 15 is transmitted to the pre-processing device 16 responding to the first pulse 31A1 of the clock pulse 30 (shown in FIG. 3).
Then the group of charge stored in the second potential-energy well a2 of the CCD shift register 15 is transmitted to the pre-processing device 16 responding to the second pulse 31A2 of the clock pulse 30 (shown in FIG. 3). Finally the group of charge stored in the n'th potential-energy well an of the CCD shift register 15 is transmitted to the pre-processing device 16 responding to the n'th pulse 31An of the clock pulse 30 (shown in FIG. 3). For the operation mentioned above, it is designed that after the n'th pulse 31An of the clock pulse 30 has been arrived at the CCD shift register 15, the pulse 23 of the pulse 20 arrives at the photo-sensing device 14. So the exposing time of the photo-sensing device 14 is a fixed value, i.e., time interval between pulse 21 and pulse 23, which is a multiplication of pixel rate and pixel number, in spite of the variation of operational mode.
The pixel rate mentioned above is the number of group of charge stored in the potential-energy well of the CCD shift register 15 in a unit time interval. The pixel number mentioned above is the number of the potential-energy well of the CCD shift register 15. In a high resolution mode, more cells of the photo-sensing device 14 are utilized to be exposed to the light source 10. Whereas, in a low resolution mode, less cells of the photo-sensing device 14 are utilized to expose to the light source 10. In addition, the lens seat 18 is moved to a position to fit the scope of projection to the photo-sensing device 14. The position of the lens 13 and the lens seat 18 in the low resolution mode which employing less cells of photo-sensing device 14 is not illustrated in FIG. 1. However the necessary exposing time interval employed in the high resolution mode is the same as that of the low resolution mode. So the user has to wait for a while even the low resolution mode of the image processing system is employed. This is an origin of waste of time for the user.
After the electrical signal has been fed to the pre-processing device 16, the dc-gain voltage amplifier 16a adjusts the dc-gain of the electrical signal and then feed it to the ADC 16b. Contrast adjustment by a Gamma characteristic is performed by the post-processing means 17, and then obtained the output signal which can be further processed or displayed.
In a traditional image-acquiring device of a modern image processing system, it is necessary to provide the user with the high resolution mode and the low resolution mode for various applications. Fewer cells of photo-sensing device 14 are exposed to the light source 10 in the low resolution mode than the high resolution mode. However, the charges in each cell of the photo-sensing device 14 are transmitted through the CCD shift register 15 to the pre-processing device 16 in both high resolution mode and low resolution mode. So the necessary exposing timing interval for the high resolution mode and the low resolution mode is all the same, which is the multiplication of the pixel rate and the pixel number of the photo-sensing device 14. And this is the waste of time for the user when a lower resolution mode of the image processing system is employed. In the traditional image-acquiring device of a modern image processing system, when the frequency of the output signal of the shift control signal source is increased to lower the exposing timing interval. It tends to result the residual charges generated in the previous exposing timing interval in the CCD shift register 15, which affects the charges in the cells of the photo-sensing device 14 in the next exposing timing interval. So the quality of the output image is damaged in the prior art image processing system.